This invention relates to a piston and a process of providing wear-resistant surfaces in the ring grooves or at least in the uppermost ring groove, preferably on the side faces of the ring grooves, in internal combustion engine pistons consisting of aluminum alloys for pistons, by arc welding, such as metal electrode inert gas welding and plasma arc welding.
In certain known internal combustion engines the ring grooves are or are likely to be subjected to particularly heavy wear, especially in the uppermost ring groove, and this heavy wear, which is due to the design of the engine or the location where it is used, may result in breakage of the piston rings.
Such heavy wear is likely to occur if the overall design of the internal combustion engine involves abnormally high temperatures in the ring zone or when fuel leaving an extremely large amount of residues is used, or when a large piston clearance has been selected for special reasons so that tilting motions of the piston are permitted, or when the air cannot be adequately cleaned, particularly in a highly dust-laden environment, e.g., on building sites.
So-called ring carrier pistons are used in such cases and comprise a piston body, which consists of an aluminum alloy for pistons and in which a ring carrier consisting of ferrous material and formed with the machined piston ring groove is mounted on the piston body at least adjacent to the uppermost piston ring and is embedded in a bonding layer consisting of an intermetallic compound (Fachkunde Kraftfahrttechnik, 2nd edition, Holland+Josenhans-Verlag, Stuttgart 1982, page 27). The embedded ring carrier involves an undesirable increase of the weight of the aluminum piston.
In a piston made of an aluminum alloy it is also known to machine a groove in the region to be provided with the piston rings and to provide the surfaces of said groove by metal spraying with a thin interlayer consisting of molybdenum, titanium, cobalt, nickel or an alloy thereof or of stainless steel and thereafter to fill said groove with a stainless steel having a high chromium content and a specified nickel content, whereafter the ring grooves are formed (Laid-open German Application No. 14 00 115). That practice has the disadvantage that there is only a poor bond between the aluminum alloy and the sprayed interlayer.
In order to improve the wear resistance of the surfaces of an aluminum alloy piston, U.S. Pat. No. 3,285,717 proposes to form a coating consisting of a wear-resistant aluminum alloy composed of 12 to 30% silicon, 10 to 30% copper, 2 to 6% manganese and optionally up to 6% iron, 0.5 to 5% nickel, 0.5 to 3% chromium and 0.5 to 1% of various other metals, such as titanium, vanadium, tungsten, molybdenum etc., balance aluminum, in a plasma arc welding process in which the aluminum alloy filler is liquefied in an electric arc and impinges on the fused surface of the piston material so as to form a hard surface layer thereon. The composition of the surface layer changes from the outside to the inside in such a manner that the layer which contains the least filler material is underneath. The alloy is formed by the diffusion of the aluminum alloy of the filler into the surface of the aluminum piston.
German Patent Publication No. 22 00 033 describes a process by which the wear resistance particularly in the uppermost ring groove of aluminum alloy pistons for internal combustion engines is improved in that an aluminum alloy containing 18 to 30% silicon and 2 to 6% copper is fusion-joined to the aluminum alloy of the piston by electron beams so that the concentration of alloying elements is increased. Such processes have not been successful because the expenditure involved in making the wear-resisting layer is excessive, as a rule.